1. Field of the Invention
The present invention relates to a scanning image-forming lens system using a rotary polygon mirror and an optical scanner using the scanning image-forming lens system.
2. Description of the Related Art
An optical scanner is well known in relation to a laser printer, a digital copying machine, etc. Recently, a high optical scanning performance has been required and there is a problem about stability of the diameter of a light spot formed on a scanned face. When the diameter of the light spot is changed in accordance with a change in scanning position of the light spot, a dot diameter of the optical scanner is changed. This change in dot diameter greatly prevents an optical scanning operation from being performed with high performance.
As is well known, the diameter of the light spot is changed by the field curvature of a scanning image-forming lens system for forming the light spot on the scanned face. In general, one of developing targets for the scanning image-forming lens system is to reduce the field curvature thereof as much as possible.
There is a problem about a so-called reflecting face inclination in a rotary polygon mirror widely used as a means for deflecting a light beam in the optical scanner. To remove an influence of this reflecting face inclination, a light beam incident to the rotary polygon mirror is focused and formed as a linear image extending in a main scan-corresponding direction in the vicinity of a deflecting reflecting face of the rotary polygon mirror. Positions of the deflecting reflecting face and the scanned face are generally set by the scanning image-forming lens system in a conjugate relation in geometrical optics with respect to a cross scan-corresponding direction.
In such a case, a forming position of the above linear image and the position of the deflecting reflecting face are slightly shifted two-dimensionally in accordance with rotation of the rotary polygon mirror. Accordingly, no field curvature of the scanning image-forming lens system is symmetrically caused on the positive and negative sides of an angle of deflection of a deflected light beam. Therefore, it is complicated to correct the field curvature of the scanning image-forming lens system.
Japanese Patent Application Laying Open (KOKAI) No. 1-92717 shows a method for correcting such field curvature in consideration of asymmetry thereof. In this method, a scanning image-forming lens system is designed such that the entire lens system is shifted from a reference optical axis in a main scan-corresponding direction and a shifted position of this lens system is set to a reference position.
This method is a field curvature correcting method utilizing a nature in which the field curvature is rotationally changed by the above shift in position of the scanning image-forming lens system. This method is effective to correct one of field curvatures in the main and cross scan-corresponding directions. However, it is impossible to simultaneously and independently correct both the field curvatures in the main and cross scan-corresponding directions.